Optical techniques for remotely characterizing fluid suspended particles are receiving increased attention in the industry because they are highly suitable for high quality measurements in a non invasive and quasi real time fashion. Most common are transmittance based measurement techniques, in which light is directed through the fluid and the directly emitting remaining portion of the incident light is used to characterize the suspended particles. Transmittance based measurement techniques are less sensitive to noise especially at short wavelengths in the UV regime but are well suitable to obtain information about particles. However, information about particle sizes and particle distributions, and n and k are difficult to obtain with transmittance measurement alone as is well known in the art. This is mainly due to the strong correlations between n, k, size, and density of particles in the analysis. For example, increasing the particle density has a similar effect, to certain extent, as increasing the particle size to the transmittance spectra. Therefore, there exists a need for an optical measurement technique for fluid suspended particles that is capable of providing information about particle sizes and particle distributions. The present invention addresses this need.
Increasing attention has been received lately regarding scattered light measurement techniques to derive information about fluid suspended particles. In combination with well known mathematical algorithms and methods that are preferably computationally implemented, light scattering measurements may require relatively small optical design effort while providing highly accurate measurement results. A low number of lenses, mirrors and optical fibers are employed to direct a broadband spectral light source onto the fluid suspended particles and collect and direct the scattered light towards a detector. Also, the measurement has characteristics that can be computationally analyzed to determine particle size and n and k. One such well known characteristic is a wavelength dependent oscillation of the scattered light. From such oscillations, n and k may be roughly computationally determined. Unfortunately, scattering oscillations decrease with particle size such that at approximately 0.2 μm particle sizes and below, n and k cannot be reliably determined from the scattered light at a fixed scattering angle. Therefore, there exists a need for an optical measurement technique for fluid suspended particles that provides information of n and k of particles also substantially below 0.2 μm size. The present invention also addresses this need.
In the prior art, scattered light is detected either by a single stationary or a single moveable detector or a number of stationary detectors. The more scattered light is detected as a function of angle, the more information about the suspended particles may be derived. Of particular interest are variations of the scattered light in relation to the scattering angle. Such variations can be detected in a continuous fashion over an extended scattering angle range. Prior art stationary detector(s) to the contrary provide simultaneous detection of only a fraction of the scattered light near a given scattering angle. Prior art moveable detectors may be continuously moved around the scatter origin but only along a linear path, which amounts also only to a small fraction of the total scattered light within a predetermined measurement range. Also, a moveable detector or a configuration for variable angle detection may require extensive design effort. Therefore, there exists a need for a method and apparatus capable of simultaneously detecting a substantial portion of the total scattered light along a predetermined scattering angle. The present invention also addresses this need.
Analyzing raw measurement data of transmittance spectra and scattering spectra is well known in the art. There are dispersion models such as Forouhi-Bloomer (U.S. Pat. No. 4,905,170), Cauchy, or others commonly implemented to reduce the number of variables. Other parameters in conjunction with basic formulations to perform calculations based on Beer-Lambert (BL) law are taught for example in Swanson et al, Applied Optics, 38, p. 5887, 1999; Nefedov et al, Applied Optics, 36, p. 1357, 1997, Furthermore, the extinction cross section of a single particle is also known to be calculated from Mie scattering theory as taught, for example in C. F. Boren and D. R. Huffman, “Absorption and Scattering of Light by Small Particles, Wiley, New York, 1983”. Also, single scattering conditions in the transmittance of fluid suspended particles are for example, taught in Swanson et al, Applied Optics, 38, p. 5887, 1999. Also, it has been noticed in experiments that some forward scattered light may also be detected as transmitted light like in Nefedov et al, Applied Optics, 36, p. 1357, 1997. All prior art address some isolated issues relating to analyzing fluid suspended particles, but fail to teach a combination of detector choice and apparatus setup, and fail to teach a relevant analytical solution to address the need for simultaneously determining refractive index, extinction coefficient, particle size and particle density. The present invention also addresses this need.